How Microorganisms Affect the Sensorial Qualities of Olive Oil

Yeasts are among the microorganisms present in olive oil and, depending on their enzymatic activities, they can either improve or damage the oil quality.

Jan. 24, 2018
By Daniel Dawson

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Microorganisms play a larger role than pre­vi­ously thought in affect­ing the chem­i­cal and sen­sory qual­i­ties of olive oil, research from the University of Molise sug­gests.

Yeasts are among the microor­gan­isms present in olive oil and, depend­ing on their enzy­matic activ­i­ties, they can either improve or dam­age the oil qual­ity. Some act as preser­v­a­tives, increas­ing the shelf life of the olive oil. Others act as cat­a­lysts for chem­i­cal reac­tions that lead to ran­cid­ity in the oil.

Certain oil-born yeast species may exert a pos­i­tive effect on the sen­so­r­ial char­ac­ter­is­tics of newly pro­duced vir­gin olive oil by pro­mot­ing the deb­it­ter­ing process.- Biagi Zullo, University of Molise

The activ­ity of healthy olive micro­biota greatly con­tributes to the preser­va­tion of the over­all qual­ity of olive oil pro­duc­tion dur­ing the post-har­vest stor­age, pro­cess­ing phase and stor­age of the pro­duced olive oil,” researcher Biagi Zullo wrote in the report.

Gino Ciafardini, a lead­ing expert on yeast in olive oil, and Zullo have been study­ing microor­gan­isms in olive oil since 2002. In the past 16 years, they have dis­cov­ered 17 species of yeast and are cur­rently fig­ur­ing out how these microor­gan­isms affect sen­sory char­ac­ter­is­tics of the oil.

Certain oil-born yeast species may exert a pos­i­tive effect on the sen­so­r­ial char­ac­ter­is­tics of newly pro­duced vir­gin olive oil by pro­mot­ing the deb­it­ter­ing process,” Zullo wrote. Other yeasts are con­sid­ered harm­ful as they can dam­age the qual­ity of the oil through hydrol­y­sis of tri­a­cyl­glyc­erols and the pro­duc­tion of unpleas­ant fla­vors.”


The num­ber and type of yeast species are heav­ily depen­dent on the chem­i­cal com­po­si­tion of the olive oil. Oils with higher con­cen­tra­tions of phe­no­lic com­pounds tend to have fewer types of yeast in lower con­cen­tra­tions.

The con­tent of the total polar phe­nols of the olive oil and their pro­file as antimi­cro­bial com­pounds, show a strong selec­tive pres­sure on the type and num­ber of years that sur­vive in the oil dur­ing its stor­age,” Ciafardini said. In gen­eral, the olive oil’s micro­bi­o­log­i­cal qual­ity is cor­re­lated with the chem­i­cal and sen­so­r­ial qual­ity of the prod­uct.”

Yeast and other microor­gan­isms that are found in the olive oil come mainly from the fruit, itself. When the olives are pressed into a paste, these microor­gan­isms become sus­pended in the oil droplets.

Previously, it was widely believed that vir­gin olive oil pro­vided an unsuit­able habi­tat for many microor­gan­isms,” Zullo wrote. Microbiological research stud­ies have since estab­lished that freshly pro­duced vir­gin olive oil is con­t­a­m­i­nated by microor­gan­isms con­sist­ing mainly of yeast that are capa­ble of con­di­tion­ing the physic­o­chem­i­cal and sen­so­r­ial char­ac­ter­is­tics of the oil.”

Ciafardini and Zullo are cur­rently cat­e­go­riz­ing these yeast species based on their pos­i­tive or neg­a­tive effects on the oil qual­ity. Ciafardini said there is no way to con­trol the types of yeast species that end up in cer­tain olive oils.

Through the olive oil fil­tra­tion it is not pos­si­ble to sep­a­rate the yeasts that poten­tially can harm olive oil qual­ity from those that help oil qual­ity,” he said. The best way is to pro­duce vir­gin olive oil of good qual­ity, obtained from healthy fruits, hav­ing suf­fi­cient polar phe­nols con­tent able to inhibit the activ­ity of some unde­sir­able yeast enzymes.”

There may be no way to con­trol the types of yeast species in the oil, but there are vari­ables that olive oil pro­duc­ers can con­trol in order to ensure they make a higher qual­ity prod­uct.

The con­di­tion of the olives when they are processed have a big effect on the types of yeast present in the oil. For exam­ple, if the olives are begin­ning to fer­ment when they are pressed into oil, cer­tain yeast species that pro­duce ethanol and ethyl acetate will be present. These two chem­i­cals give the oil a vine­gary taste if it sits unused for too long.

Ciafardini and Zullo also found that mono­va­ri­etal olive oils tend to have fewer, some­times only one, species of yeast. Depending on the con­di­tion of the olives when they are pressed, this method of pro­duc­tion dras­ti­cally decreases the chances of harm­ful yeast exist­ing in the oil.

Certain essen­tial oils also can be added to olive oil in order to lower the con­cen­tra­tion of yeast and, in cer­tain cases, coun­ter­act its neg­a­tive effects. The essen­tial oils used in lemon and gar­lic fla­vored olive oils acted as espe­cially strong inhibitors for the growth of microor­gan­isms.

Ciafardini said that research on olive oil yeast has only reached the tip of the ice­berg. He and Zullo dis­cov­ered six of the 17 species just last year. The two believe that there is much more work to be done in order to fully under­stand how dif­fer­ent yeast species affect the qual­ity of the olive oil.

In vir­gin olive oil… the pres­ence and activ­ity of yeasts were demon­strated dur­ing the past two decades,” Zullo wrote. In fact, inves­ti­ga­tions explor­ing the olive oil micro­bi­ol­ogy are com­par­a­tively scarce; there­fore micro­biota in extra vir­gin olive oil remains poorly under­stood.”


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